The Martin Company effectively scaled up their successful PBM Marinerpatrol bomber design to produce the prototype XPB2M-1 Mars.[1] After flight tests with the XPB2M between 1941 and 1943, she was passed on to the Navy. The original patrol bomber concept was considered obsolete by this time, and the Mars was converted into a transport aircraft designated the XPB2M-1R. The Navy was satisfied with the performance, and ordered 20 of the modified JRM-1 Mars.[1] The first, named Hawaii Mars, was delivered in June 1945, but the with the end of World War II the Navy scaled back their order, buying only the five aircraft which were then on the production line.[2] Though the original Hawaii Mars was lost in an accident on Chesapeake Bay, the other 5 Mars were completed, and the last delivered in 1947.

Named the Marianas Mars, Philippine Mars, Marshall Mars, Caroline Mars, and a second Hawaii Mars, the 5 production Mars aircraft entered service ferrying cargo to Hawaii and the Pacific Islands. The last production airplane (the Caroline Mars) was designated JRM-2, powered by 3,000 HP Pratt & Whitney R-4360 engines, and featured a higher maximum weight and other improvements. On April 5, 1950, the Marshall Mars was lost near Hawaii when an engine fire consumed the airplane after her crew had evacuated. The remaining "Big Four" flew record amounts of Naval cargo on the San Francisco-Honolulu route efficiently until 1956, when they were parked at NAS Alameda.[1]